1. Field of the Invention
The present invention relates to a magnetic encoder for use in a rotation detecting device for detecting the number of revolutions of a rotating element of a bearing relative to a non-rotating element thereof and also to a wheel support bearing assembly utilizing such magnetic encoder. More specifically, the present invention relates to the magnetic encoder used as a component of a bearing sealing unit mounted on a rotation detecting device that is employed in association with, for example, an automobile anti-skid brake control system for detecting respective numbers of revolutions of automobile front and rear wheels and also to the wheel support bearing assembly utilizing such magnetic encoder.
2. Description of the Prior Art
The rotation detecting device for use in association with the anti-skid brake control system for minimizing skidding of an automotive vehicle on a road surface has hitherto been assembled in a number of structures. Of them, the following structure is largely employed in practice. Specifically, the conventional rotation detecting device includes a serrated rotor and a detecting sensor, which are arranged spaced a distance from each other outside a sealing device used to seal the bearing, but are functionally integrated together to define a single and independent rotation detecting device.
This conventional rotation detecting device is of a design, in which the detecting sensor secured to a knuckle can detect the number of revolutions of the serrated rotor mounted on a rotary shaft for rotation together therewith. The bearing assembly utilizing such rotation detecting device is protected from ingress of water components and/or any other foreign matters by means of the independent sealing device positioned laterally of the rotation detecting device.
Another conventional rotation detecting device is also known, which is designed to form a part of a bearing sealing device for the purpose of increase of detecting performance and reduction of the space for installation of the rotation detecting device. As shown in FIGS. 17A and 17B, this rotation detecting device for the detection of the wheel revolution includes a magnetic encoder 50 having a multipolar rubber magnet 60. This magnetic encoder 50 is formed by vulcanizing an elastic member (rubber material) mixed with a powdery magnetic material, molding the vulcanized elastic member to represent an annular shape, bonding the annular elastic member to a generally annular slinger 61 employed in the rotation detecting device and finally magnetizing the bonded annular elastic member to have a plurality of circumferentially alternating opposite magnetic poles. See, for example, the Japanese Patent No. 2816783.
A further conventional rotation detecting device is suggested in, for example, the Japanese Laid-open Patent Publication No. 2004-084925, in which an annular multipolar magnet having a plurality of circumferentially alternating opposite magnetic poles formed therein is supported by a core metal to form a magnetic encoder. In this conventional rotation detecting device, the multipolar magnet is made of a sintered element formed by sintering a mixture of a powdery magnetic material and a binder of a powdery non-magnetic metallic material.
The magnetic encoder made of the sintered element can contain a large proportion of the powdery magnetic material as compared with that in the prior art rubber magnet and can also have a high magnetic force per unitary volume, and accordingly, not only can the detecting sensitivity be increased, but also the magnetic encoder can have a thin-walled structure. Also, as compared with the magnetic member prepared by sintering only the powdery magnetic material, cracking would hardly occur because of the presence of the powdery non-magnetic metallic material used as the binder. In addition, since the surface hardness of the multipolar magnet prepared from the sintered element is so high, as compared with that exhibited by the rubber magnet, that the multipolar magnet is hardly damaged, resulting in increase of the durability and the reliability.
According to the second mentioned patent literature, the multipolar magnet is formed by fixing the sintered element to the core metal or the slinger by means of a staking technique and then magnetizing the sintered element to have a plurality of circumferentially alternating opposite magnetic poles. In general, however, as a method of magnetizing the elastic member to form the multipolar rubber magnet 60 disclosed in the first mentioned patent literature or magnetizing the sintered element to form the multipolar magnet disclosed in the second mentioned patent literature, either one of one-shot magnetization process and index magnetization process is employed. Also, for the powdery magnetic material, a powdery ferrite material or a powdery mixture of samarium and neodymium materials is generally employed.
In the case of the magnetic encoder of the structure in which the rubber magnet is bonded to the slinger such as disclosed in the first mentioned patent literature, the elastic member must have an increased wall thickness if the magnetic flux density is desired to be increased, and since the space available in the vicinity of the magnetic encoder is limited, it is often encountered with difficulty in designing. Also, a problem has been found that collision with small gravel stones results in damages to the surface of the elastic member, accompanied by degradation of the magnetic characteristic such as reduction in magnetic flux density and increase of the pitch error to such an extent as to result in deterioration of the sensing function.
On the other hand, in the case of the magnetic encoder of the structure in which the multipolar magnet supported by the core metal is prepared from the sintered element such as disclosed in the second mentioned patent literature, not only can increase in sensitivity and reduction in wall thickness be accomplished, but also the surface hardness is high enough to avoid damage. The presence of the powdery non-magnetic metallic material used as the binder renders cracking or breaking to occur relatively hardly. However, the prevention of cracking is still insufficient and, when the magnetic encoder is press-fitted onto an inner race of a wheel support bearing, cracking tends to occur in the multipolar magnet unless careful handling is exercised. Once crack damage occurs in the multipolar magnet, rusting may occur and the pitch precision may decrease, resulting in a high risk of the sensing function being reduced.
The magnetization process will be discussed in more detail. Where the multipolar magnet 60 of the magnetic encoder 50 of FIGS. 17A and 17B disclosed in the first mentioned patent literature is magnetized by one-shot magnetization, such magnetization is carried out while as shown in FIG. 18, a multipolar magnet material (elastic member) 60A bonded by vulcanization to the slinger 61 is overlapped on magnetizing surface areas 44 formed so as to be deployed over a surface of a ring-shaped magnetizing yoke 42. At this time, when an electric current is supplied to a coil which is wound so as to surround the magnetizing surface areas 44, a magnetic field is developed to magnetize the multipolar magnet material 60A to form the multipolar magnet 60 as shown in FIG. 17A.
In such case, the slinger 61 is of a shape including a cylindrical wall 61a and a radial upright wall 61b protruding radially outwardly from one end of the cylindrical wall 61a. The multipolar magnet material 60A is positioned on one of the circumferential surfaces of the radial upright wall 61b opposite to the cylindrical wall 61a and, accordingly, the cylindrical wall 61a of the slinger 61 will not disturb the one-shot magnetization, as shown in FIG. 18. However, in this magnetic encoder 50, the sum of the width of the cylindrical wall 61a and the width of the multipolar magnet 60 represents the width of the magnetic encoder 50 itself, involving a problem associated with increase of the width.
If in order to alleviate the problem discussed above, the cylindrical wall is formed so as to protrude in the reverse direction as shown by 61aa in FIG. 17B, that is, the multipolar magnet 60 is positioned on one of the circumferential surfaces of the radial upright wall 61b adjacent the cylindrical wall 61aa, the width of the magnetic encoder 50 can be reduced by a quantity corresponding to the width of the multipolar magnet 60. However, in such case, as shown in FIG. 18, the cylindrical wall 61aa of the slinger 61 will disturb the one-shot magnetization with the multipolar magnet material 60A consequently failing to contact the magnetizing surface areas 44 of the magnetizing yoke 42.
Also, if the cylindrical wall 61aa is positioned along an inner peripheral surface (inner diameter surface) of the magnetizing yoke 42 in an attempt to render the multipolar magnet material 60A to contact the magnetizing surface areas 44, a portion of the multipolar magnet material 60A will depart from the magnetizing surface areas 44. On the other hand, if the length of the multipolar magnet material 60A is reduced to allow the multipolar magnet material 60A in its entirety to be overlapped over the magnetizing surface areas 44, there is a risk that no predetermined magnetic characteristic can be obtained at a required location of the magnet material 60A.
The foregoing problems may be substantially eliminated if the multipolar magnet material 60A is magnetized by index magnetization process. In such case, regardless of the shape of the slinger 61, the multipolar magnet material 60A can be magnetized satisfactorily. However, the use of the index magnetization process will pose the following problems depending on the kind of magnetic material used:                (1) Where ferrite is used as the magnetic material, the magnetization can be achieved, but the multipolar magnet 60 must have an increased wall thickness in order to increase the magnetic flux density, resulting in difficulty in installing the resultant magnetic encoder depending on the space available in the vicinity of where it must be disposed.        (2) Where a rare earth metal such as samarium and neodymium is employed as the magnetic material, extremely strong magnetic field must be developed to magnetize it since the rare earth metal has a high coercive force. With the index magnetization, it is not possible to develop the required quantity of magnetic field and, therefore, no satisfactorily magnetization can be achieved.        